In this article

Xamarin only actively supports the current stable release of the
Xamarin tools. The information below is provided "as-is" for older
versions of the tools. For the latest information on Xamarin releases,
please check here.

"Missing android.jar for API Level 21" in Android L Preview

This message means that the Android SDK platform for API Level 21 is
not installed. Either install it in the Android SDK Manager (Tools >
Open Android SDK Manager...), or change your Xamarin.Android project to
target an API version that is installed.

There are a few workarounds for this issue:

Change your project so that it targets API 19 or lower.

Rename your android-21 folder from android-21 to android-L. (At
best, this should only be used as a temporary fix, and it might not
work very well at all.)

Extract [1] into C:\Users\\AppData\Local\Android\android-sdk\platforms
to create an android-L folder.

The following error message (or similar) may show up:

/Library/Frameworks/Mono.framework/External/xbuild/Xamarin/Android/Xamarin.Android.Common.targets:
Error: Could not find android.jar for API Level 21.**

This means that the Android SDK platform for API Level 21 is not
installed.Either install it in the Android SDK Manager (Tools > SDK
Manager...), or change your Xamarin.Android project to target an API
version that is installed.

There are a few workarounds for this issue:

Change your project so that it targets API 19 or lower.

Rename your android-21 folder from android-21 to android-L. (At
best, this should only be used as a temporary fix, and it might not
work very well at all.)